Fuel cell systems frequently include a plurality of fuel cells assembled together to form a fuel cell stack. Proton Exchange Membrane (PEM) type fuel cells normally include an anode, a cathode and a membrane separating the anode and cathode. Each of the anodes of the individual fuel cells are both electrically and fluidly connected together such that the plurality of anodes are commonly referred to as the anode side of the fuel cell stack. Similarly, each of the cathodes of the individual fuel cells are both electrically and fluidly connected together and commonly referred to as the cathode side of the fuel cell stack. The anode side of the fuel cell stack includes an inlet for a fuel and an outlet for the non-consumed fuel and for exhaust gases which are created at the anode side and exhausted from the fuel cell stack. The cathode side of the fuel cell stack also includes an inlet normally used to inject a gaseous oxidizing agent such as air and an outlet used to exhaust gases which are produced at the cathode side. A compressor is commonly connected upstream of the cathode side inlet of the fuel cell stack to pressurize the gaseous oxidizing agent prior to injection.
The membranes of the individual fuel cells must be kept moist during operation of the fuel cell stack to protect the membranes from damage and to achieve the highest degree of fuel cell stack operating efficiency. During operation of a fuel cell stack, protons which originate from a hydrogen component of the fuel supplied to the anode side migrate through the humidified membranes and react at the cathode side with the oxidizing agent. The oxidizing agent is normally in the form of atmospheric oxygen. The hydrogen and oxygen combine to generate electrical power and also produce a volume of water. Water is therefore always present at the cathode side. A portion of this water during normal operation of the fuel cell stack diffuses back through the membranes of each fuel cell to the anode side of the fuel cell stack so that both sides of the membrane and both the anode and cathode sides are normally humidified. The water produced is often in excess of that required for normal humidification of the fuel cell stack, therefore an excess portion of the water is normally removed from the anode side and the cathode side and either collected for re-use or drained off.
Adequate humidification control of the fuel cell stack cannot always be maintained, however. During certain operating conditions the atmospheric air normally used for the oxidizing agent does not contain sufficient water volume and the cathode side of the fuel cells can therefore be inadequately humidified. To resolve this, common fuel cell stacks provide active humidification control of the cathode gas flow. A common form of humidification control involves the use of a humidifier upstream of the fuel cell stack which is separately supplied by a water source such as a tank. Water for these humidifiers is also commonly provided from fuel cell product water transferred from the exhaust of the fuel cell stack to the stack cathode inlet. Because water is being injected, to prevent direct impingement of water particles on the fuel cell membranes, the fluid volume is first preheated to entirely vaporize it. This additional process step increases the cost and complexity of the system. Proper flow control of the water injected into the fuel cell stack is also required which is commonly provided through the use of flow nozzles or pressure control valves. These components also increase system cost and complexity.
Humidifiers used at the stack cathode inlet are commonly of the membrane type. Membrane type humidifiers generally provide optimal efficiency at air inlet humidities of approximately 100% relative humidity (RH) plus a portion of liquid water. However, if the humidity at the humidifier inlet drops below 100% RH the efficiency of the humidifier will drop significantly. One solution to this problem is to increase the size of the humidifier, however this solution increases the component cost of the humidifier and can result in further configuration problems for the system. A lower cost and simpler solution is therefore desirable.